patch 8.2.4371: Vim9: can create a script variable from a legacy function

Problem:    Vim9: can create a script variable from a legacy function.
Solution:   Disallow creating a script variable from a function.
diff --git a/src/errors.h b/src/errors.h
index ef9e083..c5a2649 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3234,4 +3234,6 @@
 	INIT(= N_("E1267: Function name must start with a capital: %s"));
 EXTERN char e_cannot_use_s_colon_in_vim9_script_str[]
 	INIT(= N_("E1268: Cannot use s: in Vim9 script: %s"));
+EXTERN char e_cannot_create_vim9_script_variable_in_function_str[]
+	INIT(= N_("E1269: Cannot create a Vim9 script variable in a function: %s"));
 #endif